How Much Sodium Level Is Too Low? Understanding Hyponatremia

November 25, 2025 •

4 min read

The human body tightly regulates its internal fluid and electrolyte balance, with a normal blood sodium range of 135 to 145 milliequivalents per liter (mEq/L). A sodium level is considered too low when it falls below 135 mEq/L, a condition medically known as hyponatremia.

Quick Summary

A sodium level under 135 mEq/L is medically defined as hyponatremia, with severity determined by how much and how quickly the level drops. Lower and more rapid decreases can trigger cellular swelling, especially in the brain, causing serious symptoms and potentially life-threatening complications.

Key Points

Definition of Low: A blood sodium level below 135 mEq/L is medically defined as hyponatremia.
Severity Classification: Hyponatremia is categorized into mild (130-134 mEq/L), moderate (125-129 mEq/L), and severe (<125 mEq/L).
Symptom Range: Symptoms can include mild headaches and nausea to severe seizures and coma, worsening with lower and more rapid drops in sodium.
Causes of Imbalance: Causes can include excessive water intake, kidney disease, heart failure, and certain medications, disrupting the body's fluid balance.
Acute vs. Chronic: A sudden drop (acute) is more dangerous due to rapid brain swelling, while a gradual drop (chronic) is typically milder.
Emergency Symptoms: Severe symptoms like seizures, significant confusion, or unconsciousness require immediate emergency medical care.
Risk of Overcorrection: Treating chronic hyponatremia too quickly can cause severe, irreversible brain damage called osmotic demyelination syndrome.

What is Hyponatremia?

Sodium is a crucial electrolyte that plays a vital role in maintaining the body's fluid balance, nerve function, and muscle contractions. When the concentration of sodium in your blood (serum) drops below the normal range, the condition is called hyponatremia. The body's intricate regulatory mechanisms, including hormones like vasopressin and aldosterone, normally ensure that sodium and water levels remain in a healthy equilibrium. When this balance is disrupted, excess water moves into the body's cells, causing them to swell. The seriousness of hyponatremia depends largely on the level of the drop and the speed at which it occurs.

How are Low Sodium Levels Classified?

Medical professionals classify hyponatremia into different levels of severity based on the serum sodium concentration. This helps guide diagnosis and treatment protocols.

Mild Hyponatremia: This occurs when blood sodium levels are between 130 and 134 mEq/L. Many people with mild hyponatremia may not experience any noticeable symptoms.
Moderate Hyponatremia: This category includes sodium levels from 125 to 129 mEq/L. At this stage, symptoms are more likely to appear and may become more pronounced.
Severe Hyponatremia: Defined as a serum sodium concentration below 125 mEq/L. Profoundly low levels can lead to life-threatening complications, particularly if the drop happens rapidly.

Symptoms Associated with Low Sodium

The symptoms of hyponatremia can vary significantly based on its severity and whether it is acute (sudden onset) or chronic (gradual onset).

Common symptoms include:

Nausea and vomiting
Headache
Loss of energy, drowsiness, or fatigue
Muscle weakness, cramps, or spasms
Restlessness and irritability
Confusion or other mental changes

Severe symptoms, which warrant immediate emergency care, include:

Seizures
Decreased consciousness or lethargy
Coma
Hallucinations

Causes and Risk Factors for Low Sodium Levels

Hyponatremia is often the result of an imbalance between water and sodium, caused by a variety of medical and lifestyle factors.

Causes related to excess body water:

Excessive water intake: Can overwhelm the kidneys' ability to excrete water, diluting the blood sodium. This is common in endurance athletes who drink too much plain water without replacing electrolytes.
Syndrome of inappropriate antidiuretic hormone secretion (SIADH): A condition where the body produces too much antidiuretic hormone (ADH), leading to water retention.
Heart, kidney, or liver problems: Congestive heart failure, chronic kidney disease, or cirrhosis can cause fluid to accumulate, diluting the blood's sodium content.

Causes related to sodium loss:

Severe vomiting or diarrhea: Causes a loss of fluids and electrolytes, including sodium.
Diuretics: Some water pills can cause the body to excrete too much sodium.
Addison's disease: Adrenal gland insufficiency affects hormones that regulate sodium and potassium.

Other risk factors:

Older age
Certain medications (e.g., antidepressants, pain medicines)
Use of the drug Ecstasy
Post-surgery status

Comparison: Acute vs. Chronic Hyponatremia

The timeline of a sodium drop is critical for determining the immediate health risk and the treatment approach. Overly rapid correction of chronic hyponatremia can cause a severe neurological complication called osmotic demyelination syndrome (ODS).


Feature	Acute Hyponatremia	Chronic Hyponatremia
Onset	Occurs rapidly, typically within 48 hours.	Develops gradually over more than 48 hours.
Brain Adaptation	Brain cells have little time to adapt, leading to a high risk of dangerous swelling (cerebral edema).	Brain cells have time to adjust, reducing the risk of cerebral edema.
Symptoms	Often severe and sudden, including seizures and coma.	Symptoms are typically milder and may be absent, but can include fatigue and confusion.
Treatment Urgency	An emergency requiring rapid, but controlled, correction of sodium levels.	Correction is slower and more cautious to prevent osmotic demyelination syndrome.
Associated Causes	Excessive water intake, recreational drug use, or severe, sudden fluid loss.	Chronic illnesses such as heart failure, liver cirrhosis, or SIADH.

Treatment for Low Sodium Levels

Treatment for hyponatremia is highly individualized and depends on the underlying cause, severity, and duration of the condition.

Addressing the Underlying Cause: This is the most important step. For example, adjusting diuretic medication, treating heart or kidney disease, or managing SIADH.
Fluid Restriction: For cases caused by fluid overload, restricting water intake can help balance sodium levels.
Intravenous (IV) Saline: For severe, symptomatic hyponatremia, an IV solution of concentrated saline may be administered to slowly and safely raise blood sodium. This must be done with extreme caution under medical supervision to avoid overcorrection.
Medications: Certain medications, including vaptans, can help the body excrete excess water while retaining sodium.

When to Seek Medical Help

While mild hyponatremia might be asymptomatic, all cases below the normal range of 135 mEq/L should be medically evaluated. Seek immediate emergency care for anyone with severe symptoms.

Call your doctor if: You are at risk for hyponatremia (e.g., have heart or kidney disease, take diuretics) and experience nausea, headache, cramping, or fatigue.
Go to the ER immediately if: Confusion, seizures, or loss of consciousness occur.

Conclusion

Understanding how much sodium level is too low is crucial for recognizing the serious health risks of hyponatremia. The condition, defined as a blood sodium level under 135 mEq/L, can range from a mild, asymptomatic issue to a life-threatening emergency depending on its severity and speed of onset. Given the dangers of brain swelling and the complexity of treatment, any confirmed low sodium result should be addressed by a healthcare professional. Athletes and individuals with chronic diseases should be particularly vigilant in managing their fluid and electrolyte balance. For more information, consult the National Kidney Foundation's guide on low blood sodium, or consult a medical professional for advice.

National Kidney Foundation: Hyponatremia

Frequently Asked Questions

The normal range for blood sodium levels is typically between 135 and 145 milliequivalents per liter (mEq/L).

Severe, or profound, hyponatremia is defined as a serum sodium concentration below 125 mEq/L. At this level, the risk of serious complications is much higher.

Yes, drinking excessive amounts of water can cause low sodium by diluting the blood and overwhelming the kidneys' ability to excrete water.

The most dangerous effect is brain swelling, known as cerebral edema. When sodium levels drop rapidly, water moves into the brain cells, causing them to swell, which can lead to seizures, coma, and even death.

Treatment varies depending on the cause and severity. It may involve restricting fluid intake, adjusting medications, or, in severe cases, administering IV fluids with sodium under careful medical supervision.

You should go to the emergency room immediately if you experience severe symptoms like confusion, seizures, loss of consciousness, a severe headache, or repeated vomiting.

Acute hyponatremia develops suddenly (within 48 hours), posing a higher risk of rapid brain swelling. Chronic hyponatremia develops gradually, allowing the brain to adapt, making symptoms generally milder.

Osmotic demyelination syndrome (ODS) is a severe neurological complication that can occur if chronic hyponatremia is corrected too rapidly. It causes damage to the brain stem and can lead to permanent disability.